You meant Crypt? It isn't white spot in saltwater.
I would do fresh water for the water change. Are you pulling all your fish from your DT? If not, you might as well not waste your time because these *bugs* will still be in there.
If you are pulling all the fish you have to treat, then you need to leave your tank fallow.
Here is from wetwebmedia.com
After Observing Infestation:
Many products, chemicals have been advanced as being efficacious in treating for Crypt, some in combination with others. In general the more effective treatments are more potentially toxic... all are and their mis-use is likely more a source of mortality than the actual parasites they're being used to eliminate. Be aware that there are a few commercial "reef safe" remedies (pepper-sauce, garlic...) on the market that are unreliable to put it mildly. Rather than saving fish lives these persistent "cures" kill-off hobbyists by the droves. Avoid them by getting on the internet, converse with fellow hobbyists re what works and doesn't.
Temperature effects. As with freshwater ich, it's advised to raise your systems temperature to speed up the life cycle of Crypt while you're treating for it. If your livestock can handle it, increase your heating to the mid 80's F along with whatever other treatment regimen you employ.
A) Hyposalinity, lowered specific gravity. Some advocates place lower spg. as low as 1.009. This can work if your fishes are not too challenged already or the pathogen too virulent, however it will not effect a permanent system cure. Know that most common measures of specific gravity are temperature specific and that most non-fish livestock will not tolerate the lower limit (14-16 ppt salinity) necessary to kill off the parasites. Therefore your fishes will have to be separated from your non-fish livestock if you're using hyposalinity as a treatment mode. And there are exceptions, variations to consider using hyposalinity. Cartilaginous fishes (sharks, rays) cannot be treated in this fashion... and such osmotic changes need to be made gradually (over days).
B) Ionic copper solutions, chelated and not. Copper is an old-timey, but proven method of eliminating Cryptocaryon. Solutions come in two varieties, bound up with a "carrier" molecule (chelated) and "free" (as in copper sulfate solutions). Both types have their benefits and shortcomings. Chelated copper "lasts longer" in marine water, cutting down the frequency of administration, whereas free copper is more available, readily effective. Note that you need to have/use a test kit for either type of copper used and that they are different test kits. Whichever format of copper is utilized it should be tested for and if necessary added to twice or more often daily. Often testing, adjusting copper levels assures that a "physiological dose", sufficient concentration (0.15-0.25 ppm over 7-10 days plus) of cupric ion is present to kill the tomite/theront stages.
C) Metronidazole (aka Flagyl), Quinacrine Hydrochloride, Quinine Sulfate. Not effective consistently.
D) Formalin or formalin/malachite or formalin/copper mixtures. Can be useful for initial infestations, treating large numbers of specimens, but the biocide formalin is dangerously toxic in the hands of the uninitiated. If used, shy on the low concentration, utilize extra aeration/circulation and closely watch your fishes and biological filtration.
E) Various schemes at moving infected fishes to new quarantine/treatment tanks, and vacuuming either bare-bottomed or systems with substrates: These approaches hope to capitalize on timing to eliminate intermediate forms of Crypt. Can be successful, but requires timeliness, effort on your part.
The Real Deal: Treating Fishes in Isolation, Allowing the Main System to "Go Fallow"
There are no "reef-safe" and effective treatments for crypt. NONE. Curing infested fishes involves separating them from non-fish livestock and treating them in that other system (or alternatively moving the non-fish livestock). Infested systems can be made "crypt-free" or better "crypt-virulence-reduced" by having them kept free of fish hosts for several (4 or more) weeks without fishes. If practical, elevating temperatures and possibly lowering specific gravity (to the tolerance of other non-fish livestock) can be employed to "speed up" the loss of virulence of the parasites. In practical terms we are generally talking the low to mid 80's F. and 1.017-1.018... with these values adjusted over days time. Care must be exercised in not possibly transmitting disease organisms from the quarantine system... on nets, containers, hands... anything wet, and drying, otherwise sterilizing quarantine tanks and gear between uses.
About Ultraviolet Sterilizers:
UV's cannot effect an actual "cure", though they do seem to do so in some cases where slight improvement in the overall system water quality may tip the balance between health/disease in the fishes side. Otherwise UV use can significantly reduce the number of free-swimming tomites. An initially improved situation is often perceived in initial infestations with a synchronized population of adults cycling off their hosts (every 3 to 7 days)... only to resurface in great numbers due to the confines of captivity.
/www.waterlife.co.uk/waterlife/protozin.htm
